There is a lower limit to the thickness of single crystal silicon solar cells manufactured with an aluminum back surface field (BSF), since the Al BSF fabrication process, which involves screen printing an Al paste, induces a bow in thin silicon wafers when the Al paste is fired. For 200 micron thick wafers bow starts to affect solar cell yield, and for wafers of 150 microns and less wafer bow becomes a yield killer for solar cell fabrication. The Al paste shrinks during firing causing the wafer to bow such that the Al covered surface becomes convex. This wafer bow may result in wafer breakage during subsequent processing, particularly during tabbing and stringing, and this is becoming a greater concern, as the solar industry migrates to larger wafers, from 125 mm to 156 mm square (or pseudosquare) wafers, for example. There is a need for a manufacturable alternative to an Al BSF for making thinner single crystal silicon solar cells.
The front-side p-n junction in single crystal silicon solar cells is currently manufactured using a diffusion process, which also requires a post-diffusion clean. There is a need for a more efficient manufacturing process which avoids front side diffusion and clean.